A micelle obtained through formation of a polyion complex of a copolymer containing PEG and cationic polyamino acid and nucleic acid is known as a micelle for nucleic acid delivery (Patent Literature 1). The micelle has a certain degree of practical utility in delivering a biochemically stable molecule such as DNA. However, the micelle has low stability in the living body, for example, and thus still has a drawback in delivery efficiency for a molecule having low stability such as RNA.